In recent years, a vacuum brazing technique wherein no flux is required in brazing aluminum structures has been developed and is widely used because there is no fear of pollution. In the vacuum brazing technique a brazing sheet comprised of an aluminum alloy core material having one or both surfaces thereof clad with an aluminum alloy brazing material skin is used, and the brazing is carried out in a vacuum. Vacuum brazing is used in brazing various structures, including heat exchangers. For vacuum brazing sheets, various aluminum alloy core materials and various aluminum alloy brazing skin materials have been developed and are standardized at present by JIS (Japan Industrial Standard) Z3263.
For a vacuum brazing sheet used in heat exchangers, generally JIS A3003, JIS A3005, JIS A3105, or JIS A6951 alloy is used as a core material, and JIS BA4004 or JIS AA4104 alloy brazing material having the composition shown in Table 1 is used as a skin material. Brazing sheets that have a thickness of 0.5 to 1.2 mm, one or both of whose surfaces are clad with a brazing material, with the cladding ratio being 5 to 15% on each surface, are used.
TABLE 1 __________________________________________________________________________ Alloy Name Si Fe Cu Mn Mg Zn Bi Al __________________________________________________________________________ 4004 9.0- 0.8 0.25 0.10 1.0- 0.20 -- Balance 10.5 or or or 2.0 or below below below below 4104 9.0- 0.8 0.25 0.10 1.0- 0.20 0.02- Balance 10.5 or or or 2.0 or 0.20 below below below below (wt %) __________________________________________________________________________
As aluminum heat exchangers that have a hollow structure wherein such vacuum brazing sheets are used there are, for example, drawn-cup-type evaporators, oil coolers, and radiators are produced. For instance, a drawn-cup-type evaporator, such as shown in FIG. 3, is manufactured by stacking members (1), arranging a corrugated fin (2) between the members (1), and brazing them by heating at 600.degree. C. in a vacuum on the order of 10.sup.-5 to 10.sup.-4 Torr. In this vacuum brazing, Mg added into the brazing material (e.g., JIS BA4004 alloy and JIS AA4104 alloy) of the brazing sheet evaporates gradually from about 400.degree. C., and serves to remove in-oven oxidizing gases by the getter reaction of EQU H.sub.2 O+Mg.fwdarw.MgO+H.sub.2 and O.sub.2 +2Mg.fwdarw.2MgO.
The Mg in the brazing material also evaporates radically when the brazing material is melted, so that the oxide film on the brazing material surface is broken, thereby making brazing possible. Si works to lower the melting point of the solder, thereby lowering the brazing temperature and making the brazing ability favorable.
On the other hand, when an oven is used for a long period of time, it is inevitable that the degree of vacuum decreases (e.g., to the order of 10.sup.-3 Torr) due, for example, to contamination of the oven. In such a case, the wettability of the solder of the brazing sheet lowers, fillet of the solder or brazing material is not formed, and the brazing ability lowers. That is, a failure phenomenon, i.e., the so-called braze-joint-defect, occurs. Therefore, development of Al--Mg--Si alloy brazing materials and brazing sheets excellent in brazing ability is desired.
Therefore, an object of the present invention is to provide an Al--Mg--Si alloy brazing material suitable as a skin material of brazing sheets in the vacuum brazing technique.
Another object of the present invention is to provide a brazing sheet to be used in the vacuum brazing technique, wherein the brazing material comprises an Al--Mg--Si alloy and stable and excellent brazing ability is exhibited without being influenced by a change in brazing conditions, such as lowering of the degree of vacuum.
Still another object of the present invention is to provide a brazing sheet excellent in vacuum brazing ability suitable for the manufacture of aluminum heat exchangers that have a hollow structure.